1. Field of the Invention
The present invention relates to a current supply circuit, a current-controlled type ring oscillator, a nonvolatile semiconductor memory device using these elements and an electronic card and electronic device using these and is used for, for example, a NAND flash memory and its applied devices.
2. Description of the Related Art
A semiconductor device sometimes has a built-in ring oscillator for generating a clock for use in an internal timer circuit, booster circuit and so on.
FIG. 20 shows one example of a conventional ring oscillator incorporated in the semiconductor device.
The ring oscillator is comprised of an odd number stage of inverters IV arranged as a ring-like array in which the inverter IV is used as a unit delay element. The oscillation period Tosc of the ring oscillator is determined by the stage number n of the inverters and a rising time tph and falling time tpi of an output signal of each inverter and can be represented byTosc=n×(tph+tpl)
It is desirable that the clock signal of the timer circuit, booster circuit, etc., be set to have an accurate period not depending upon the power supply voltage, threshold value of a transistor and so on or the period can be controlled. Further, it is desirable that, in order to heighten the voltage boosting efficiency of the booster circuit, the clock signal of the booster circuit be set to have a period which becomes shorter as the power supply voltage is lowered and longer as the power supply voltage is raised.
Since, however, in the ring oscillator with the inverter IV used as a unit delay element, the rising time tph and falling time tpi of the output signal of the inverter IV become shorter as the power supply voltage is raised, the period of the generating clock signal becomes shorter as the power supply voltage is raised.
FIG. 21 shows a characteristic curve revealing one example of a power supply voltage dependence of the oscillation period Tosc of the ring oscillator shown in FIG. 20.
Although, in a region of the power supply voltage which is higher than 2V, less power supply voltage dependence of the oscillation period Tosc is involved, when the power supply voltage falls from 2V, the oscillation period Tosc becomes abruptly longer, that is, the oscillation frequency is abruptly lowered.
In the case where the output of the ring oscillator shown in FIG. 20 is used as a clock signal of the booster circuit, if the booster circuit is optimized to a lower voltage operation, the voltage boosting capability of the booster circuit becomes too high at a high voltage operation time and, for example, a ripple problem is involved.
Since the rising time tph and falling time tpi of the output signal of the inverter in the ring oscillator shown in FIG. 20 depend upon the threshold value of the transistor, it is difficult to control the oscillation period Tosc (the period of the generating clock signal) when taking a variation of the threshold value into consideration.
FIG. 22 shows another conventional example of a ring oscillator incorporated into the semiconductor device.
This ring oscillator is comprised of an odd number stage of differential amplifiers 90 arranged in a ring-like array in which the differential amplifier 90 operated at a constant current is used as a unit delay element.
Each differential amplifier 90 comprises a pair of transistors N1, N2 supplied with a complementary signal from a preceding stage differential amplifier 90, load resistors Rr each connected between a power supply node and the drain of the corresponding transistor (N1, N2), a current supply NMOS transistor N3 connected between a grounded node and a source common-connected node of the transistors N1, N2 and having its gate supplied with a bias voltage biasn to allow a flow of a constant current Ir, and an output capacitance Cr including a parasitic capacitance.
The delay time of the differential amplifier 90 is determined by a constant current Ir, load resistance Rr and output capacitance Cr including a parasitic capacitance. As the current Ir becomes larger, resistance Rr becomes smaller and capacitance Cr becomes smaller, the delay time becomes shorter. By varying these parameters it is possible to vary the period of a clock signal generated from the ring oscillator. Further, the amplitude of a complementary clock signal which is outputted from the differential amplifier 90 is determined by Rr×Ir and does not depend upon the power supply voltage Vcc, so that it is strong against a power supply variation and lower voltage.
It is known that, in order to generate a constant current not depending upon the temperature, the current of a power supply having a positive temperature characteristic and current of a power supply having a negative temperature characteristic are additively combined with the use of a band gap reference (BGR) type reference voltage supply whereby the temperature characteristics cancel each other.
FIGS. 23, 24 and 25 show, by way of example, a BGR type current supply having a positive temperature characteristic, a current supply having a negative temperature characteristic and a temperature characteristic canceling power supply, respectively.
In FIG. 23, a current Ia having a positive temperature characteristic can be represented byIa=2kT·1nN/Ra2·q and has a positive temperature characteristic, noting that N represents an area ratio of two diodes.
In FIG. 24, a current Ib having a negative temperature characteristic can be represented byIb=VA/Rb noting that VA is determined by the characteristic of the diode and, since VA becomes lower as the temperature is raised, has a negative temperature characteristic.
If the ratio between Ia and Ib is optimally selected, it is possible to cancel these temperature characteristics by additively combining the copying current Ia corresponding to the current Ia of the current supply shown in FIG. 23 and copying current Ib corresponding to the current Ib of the current supply shown in FIG. 24. By copying the thus obtained current Ic by the current mirror circuit it is possible to generate two bias voltages nbias and pbias by two diode-connected transistors and, based on this, provide a constant current supply. The thus generated current Ic reveals no Vcc dependence and, since the bias voltages nbias and pbias vary by the threshold value of the transistor used, reveals no threshold value dependence.
If, however, the current supply operation shown in FIG. 25 is made to correspond to a current supply voltage Vcc of a broader range from, for example, about 4V to about 1V, a drain/source voltage Vds of a constant current supply transistor also varies under the power supply voltage Vcc in accordance therewith. In this case, it is better to set the IV characteristic of a pentode area of a transistor to be constant irrespective of Vds but, even in the pentode area, Ids is normally increased with an increasing Vds. That is, strictly, the constant current supply also comes to have a positive dependence with respect to the power supply voltage Vcc.
In the case where the ring oscillator as set out above is constructed with the use of such a current supply, the constant current supply has a positive power supply voltage dependence and the period of the output clock signal becomes shorter when the power supply voltage becomes higher and longer when the power supply voltage becomes lower, thus presenting a problem.
Incidentally, U.S. Pat. No. 6,414,516 specification discloses the technique of controlling the operation of a portion of a circuit constituting a ring oscillator using a difference current between a current of a constant current supply and a current depending upon a power supply voltage.